To perform whole machining of an inlet port valve seat by locating with high accuracy, whose positional accuracy is especially required in a cylinder head.

The position of a reference boss of the cylinder head, the position of an inlet port valve seat side center and the position of a boss of an inlet port valve guide are measured. The measured values and design allowable values are compared, and whether or not the measured values are in the design allowable values is decided. When the measured values are out of the design allowable values, adjusting and machining amount of the reference boss is calculated so that the measured values may be within the design allowable values. The adjusting and machining of the reference boss is performed according to the adjusting and machining amount. Subsequently, the machining is performed. The positional accuracy of the inlet port valve seat can be stably and highly maintained. The cylinder head having high positional accuracy of the inlet port valve seat can be obtained with high yield.